The present invention relates to oversampling digital acquisition systems, and more particularly to calibration of the oversampling digital acquisition systems using a calibration signal having random edges relative to a sample clock.
In a digital acquisition system oversampling often is used to increase sample resolution. It is common to use a periodically clocked flip-flop to sample a digital signal, i.e., a bi-level signal where one level represents a logical “1” and the other level represents a logical “0”. The resolution of this approach is limited by a maximum clock rate specified for the flip-flop. Oversampling overcomes this limitation by using multiple flip-flops to sample the digital signal in parallel. The clock input for each flip-flop is delayed by some fraction of the sample clock period to provide a range of sample points between sample clock edges. For example a clock period of 4.0 nanoseconds may be oversampled by eight flip-flops, with the sample clock being delayed by differing amounts—0.0 ns, 0.5 ns, 1.0 ns, . . . , 3.5 ns respectively—to produce an effective sample resolution of 500 picoseconds. The resolution of the oversampling system depends on the characteristics of the flip-flop, the number of flip-flops, the sample clock period and the accuracy of the individual clock delays.
The sample clock delay accuracy for each flip-flop is critical to sampling resolution improvement. The accuracy of these delays may be improved by including adjustable offset delays that are calibrated to minimize timing errors. This may be accomplished by sampling a calibration signal that has a period slightly longer or shorter than the sample clock period—4.01 ns or 3.99 ns relative to a 4.0 ns sample clock period. During each sample clock period the calibration signal precesses relative to the sample clock by the difference between the periods. The time difference between two oversampling points may then be calculated as the number of clock periods between transitions in the sample flip-flops multiplied by the rate of precession. This approach produces resolution on the order of picoseconds using crystal frequency references. However this approach is very sensitive to noise. Noise smears the transition of the calibration signal across many sample periods. System noise contributes two errors to actual data sampling: (i) it reduces the resolution of the sample flip-flops; and (ii) it compromises the resolution of the oversampling calibration.
What is desired is a method for calibrating an oversampling digital acquisition system that is not affected by random system noise to assure more accurate sampling time resolution.